1. Field of the invention.
This invention relates to capacitively-charged electrical devices generally and, more particularly, to means for rapidly and dynamically discharging such devices.
2. Background Art.
While the present invention is described as being applied, in one embodiment, to the discharge of the gate-source capacitance of a MOSFET and, in a more specific embodiment, to the photo-voltaically charged gate-source capacitance of a MOSFET in an optically-coupled solid state relay, it will be understood that it may be applied as well in any case in which it is desired to rapidly discharge a capacitively-charged electrical device and such is within the intent of the invention.
In one type of optically-coupled solid state relay, the relay comprises, in one of its simplest forms, a light-emitting diode (LED), a photovoltaic (PV) generator chip, and a MOSFET power transistor. The LED and the photovoltaic generator chips are optically coupled by one of several methods so that the radiation from the LED falls on the PV chip. The PV chip typically comprises a monolithic series-connected array of photo-sensitive diodes, with each diode generating about 0.5 volts and a few micro-amps in response to the radiation emitted by the LED. Since the photosensitive diodes are connected in series, the generated voltages are summed, so that the array--typically 20 diodes--will generate about 10 volts as an output. This open-circuit voltage is then applied between the gate and the source terminals of the MOSFET, thus enabling it to conduct, or "turn on".
Part of the turn-on process involves the output current of the PV-chip charging the gate-source capacitance, permitting the MOSFET conduction to start. The higher the charging current from the PV-chip, the more quickly will the turn-on process proceed. The value of the current is dependent on many mechanical and process variables, chief among which are the distance from the LED to the PV-chip and the size of the diodes in the chip.
The "turn-off" process--cessation of MOSFET conduction--involves stopping the LED illumination by reducing the LED drive current to zero. This causes the photo-voltaic generation which was maintaining the MOSFET gate-source capacitance to collapse. That capacitance must now discharge before the MOSFET conduction will actually cease, so there is a finite time delay between the instant the LED is turned off and the actual cessation of MOSFET conduction. This time delay will depend both on the size of the gate-source capacitance and the type of discharge path or circuit through which it can discharge. Without some provision for enhanced discharge, the capacitance will discharge through stray leakage paths which may require on the order of several seconds before the MOSFET is turned off.
Any discharge path provided for enhanced turn-off has the possible adverse effect of hampering the gate-source charging for turn-on. Therefore, the discharge circuit is usually a compromise between the turn-off and turn-on times required. For example, early designs simply used a resistor in the discharge path between the gate and the source, with the resistor having a value low enough for reasonably fast discharge, yet not so low as to steal a significant fraction of the charging, or turn-on, current. Practical designs employing such a resistor typically have discharge times on the order of 500 microseconds to 1 millisecond, but ranging higher or lower depending on the size of the capacitance being discharged. The resistor approach leaves much to be desired, although it is simple to apply.
An improvement over the resistor approach is described in U.S. Pat. No. 4,390,790, issued June 28, 1983, to Rodriguez. That patent describes a discharge path in which the resistor is replaced with a depletion-mode JFET. This component has the property of acting like an infinitely high value resistor when its gate terminal is activated, thus "pinching off" the conduction path in which it is connected, yet it is capable of returning to conduction when its gate terminal has had its voltage reduced to zero. The voltage needed to control the JFET gate--several volts--is generated by a second PV-diode array optically coupled to the same LED that is illuminating the main PV-diode array. This approach reduces the MOSFET discharge time somewhat, i.e., to on the order of 150 microseconds, but being higher or lower depending on the size of the capacitance being discharged. Its main limitations, however, are that the JFET gate itself has a capacitance charge which must be discharged before the JFET can return to conduction and there must be a relatively high value resistor in its discharge path, with the value of the resistor being determined by the same considerations discussed above. This structure retains a fairly significant RC product, and is thus not inherently the fastest possible discharge path. The slower the JFET gate discharges, the longer will be the delay time in discharging the MOSFET gate. Thus, the JFET gate is left to discharge in a rather passive manner, and is not driven in any sense.
Accordingly, it is a principal object of the present invention to provide means for discharge of a capacitively-charged electrical device which allows shorter discharge time than known such means.
It is another object of the present invention to provide means for the rapid discharge of the gate-source capacitance of a turned-on MOSFET.
It is a further object of the present invention to provide means for the rapid discharge of the gate-source capacitance of a turned-on MOSFET which is incorporated in a optically-coupled solid state relay.
It is an additional object of the present invention to provide such means that produces a relatively constant turn-off time, regardless of the value of the capacitance to be discharged.
It is yet another object of the present invention to provide such means that is relatively temperature independent.
It is yet a further object of the present invention to provide such means that may be easily and efficiently designed and manufactured.
Other objects of the present invention will, in part, be obvious, and will, in part, be apparent from the following description and the accompanying drawing figures.